1. Field of the Invention
The present invention relates to an apparatus for the high resolution measurement of the location of the edge of an opaque object. Specifically, the apparatus determines with high resolution the coincidence of the center of a laser beam and the edge of an opaque object when the laser beam and the edge of an opaque object are scanned past each other. More particularly, the invention relates to non-contacting electro-optical apparatus for this type of determination which is capable of rapidly measuring the dimensions of objects or holes in objects, measuring the location of edges of objects with very high resolution.
2. Prior Art
For the accurate measurement of the diameter, position, or thickness of soft, delicate, hot, or moving objects, non-contacting sensors must be used. Prior-art devices of this character include capacitive gauges, eddy-current gauges, air gauges, gamma and X-ray gauges, and optical sensors. Only the optical and nuclear gauges can work at distances greater than a small friction of an inch with sufficiently sensitivity. The nuclear gauges permit large working distances; however, they are extremely expensive and susceptible to systematic errors due to slight variations in the chemical composition of the object being measured.
Optical sensors have advantages because of the nature of light itself. The principle advantages are:
1. They do not require direct mechanical contact between the sensor and the object to be measured;
2. The distance from the sensor to the object to be measured can be large;
3. The response time is limited to that of the photodetector and its electronics;
4. Light variations are directly convertible to electrical signals; and
5. The measurements are independent of the chemical composition of the object.
Prior-art optical sensors employ various techniques to measure the location of the edge of an object and the time interval between the occurrence of two sensed edges. For example, Petrohilos U.S. Pat. No. 3,905,705 issued Sept. 16, 1975 discloses an optical measuring apparatus wherein the size of an object is measured by counting the number of constant frequency pulses which occur between two edge pulses. This traditional method of measuring time by counting cycles of a time base clock leads to the unavoidable quantization error of .+-.1 count. This error makes the technique unsuitable for very fine measurements; for example, for the measurement of optical fibers, a resolution of the order of 10 microinches is desired.
To reduce this error, there are three prior art techniques:
1. A very high frequency clock, e.g. 300-500 MHZ, can be used. While this approach is obvious and straight forward, it is costly in practice, due to the complications introduced by the high frequency.
2. A dual vernier interpolation using two high frequency clocks with slightly different frequencies can also be used, see Hewlett-Packard Journal, August 1978 (pages 2 to 11). This technique is quite complex and also costly.
3. An analog interpolation can be done by integrating and measuring a pulse of constant height, and width equal to the delay between the edge being measured and the time base. (See Electronic Design, Apr. 26, 1980, p. 137.) This technique is difficult to apply when a very short time interval must be measured.